Television or videocassette recorders have on-screen functions of displaying information that indicates the state of the device such as channels or program of recording, on the television screen.
Initially, an on-screen display will be described with reference to FIG. 6. FIG. 6 is a diagram illustrating a structure of a conventional on-screen display.
In FIG. 6, a display character setting unit 100 writes character data that is composed of a character code of a desired character to be displayed (hereinafter, referred to as display character), and an attribute code that indicates modification information, such as the color of the display character or background thereof, at a predetermined position in a video RAM 200.
The video RAM 200 has addresses corresponding to respective positions on the display on which the display characters are to be displayed. FIG. 7 shows an address map of the video RAM 200. On a screen of the display, 12 lines of 24 characters can be displayed. In addition, character data is composed of 8 bits, 7 bits of which are character codes indicating up to 128 types, and 1 bit of which is an attribute code.
A display control unit 300 reads character data from the video RAM 200, reads font data corresponding to the character code included in the read character data, from a character generator ROM 400, and outputs an on-screen output signal using the read font data.
The character generator ROM 400 contains plural kinds of font data.
A compositing unit 500 composites an input video signal and the on-screen output signal that is outputted from the display control unit 300.
FIG. 7 shows an address map of the conventional video RAM 200. Character data of from the 1st character on the 1st line to the 24th character on the 12th line are stored at respective addresses corresponding to predetermined display positions on the display. In other words, 12 lines of 4 characters can be displayed on the screen of the display. The character data is composed of 8 bits, 7 bits of which are character codes indicating up to 128 types, and 1 bit of which is an attribute code.
The operation of the on-screen display that is constituted as described above will be described hereinafter.
First, the display character setting unit 100 writes character data of one screen, which is composed of a character code and an attribute code of a character to be displayed, at an address in the video RAM 200 corresponding to a position on the display at which the character is to be displayed, as indicated by the address map of the video RAM shown in FIG. 7.
Then, the display control unit 300 reads the character data written in the video RAM 200 at a predetermined timing, and then reads font data stored at an address that is indicated by the character code included in the character data, from the character generator ROM 400. Then, the display control unit outputs an on-screen output signal on the basis of the read font data and the attribute code that is included in the character data read from the video RAM 200, in synchronization with an input video signal. Then, the compositing unit 500 composites the on-screen output signal and the video signal to output a composite signal to the display.
As described above, a character to which desired character modification has been added can be displayed on the display. In order to further add the character modification (coloring of the display character itself, the color of the background, and the like), details of the character modification may be previously stored in the video RAM 200 (see Japanese Published Patent Application No. Hei.9-54575).
However, in the above-mentioned construction, to increase the types of characters that can be displayed, the number of bits corresponding to the character code should be increased, resulting in an extension of the area in the video RAM 200 for holding the character code. Particularly in cases where the video RAM area is commonly used as a RAM area of the CPU (see Japanese Published Patent Application No. Hei.11-102352), and when the bit length of character data is larger than the bit length of data that is accessed to the RAM of the CPU, the video RAM area is doubled in size. More specifically, when the bit length of data accessed to the RAM of the CPU is 8 bits and the character code are 8 bits, the character data is composed of 9 bits including 1-bit attribute code. Therefore, two addresses are required for one character as shown in FIG. 8, thereby doubling a required video RAM area.